The method of this invention is useful for the preparation of reinforced polyester compositions. There is a continuing need in various industrial arts for improved reinforced polyester compositions which are useful as composites. Composites are those materials formed by mixing extremely strong and stiff fibers in a polymer resin matrix or binder. The materials in this class have advanced mechanical properties. The fibers that dominate the field of advanced composites are, in order of chronological developmental, glass, boron on a tungsten filament core, graphite or carbon, and aromatic polyamides. They possess the desirable properties of low density and extremely high strengths and moduli.
It is well known to those skilled in the art that composites are difficult to prepare from high molecular weight polymers because it is difficult to get adequate "wetting" or flow of the high molecular weight polymer into the reinforcing material. It is a distinct advantage, therefore, to have a low molecular weight/low melt viscosity material (such as cyclic ester oligomers) which can be melted, flowed into the reinforcing material, and, subsequently, polymerized to attain high molecular weight and good toughness.
Composites based on cyclic polymers having high molecular weights, such as polycarbonates, polyetherimides, polyarylates, and polyketones, are known in the art as reported in the Newsletter of High Performance, entitled Advanced Materials, Vol. 11, No. 16, Sep. 25, 1989.
Also, the superior improved moldability and improved properties of a molded article obtained by reinforcing poly(alkylene terephthalate) resins such as poly(ethylene terephthalate) (PET) and poly(cyclohexylene dimethylene terephthalate) (PCT) has long been established (see e.g., U.S. Pat. No. 3,814,725).
The relationship between high molecular weight and high melt viscosity is well established. See Flow Properties of Polymer Melts, 2d Ed., published in 1981 by J. A. Brydson, pp. 67-69. Compositions having high molecular weights such as those known in the art are expected to have a high melt viscosity.
As described previously, a low melt viscosity is required in the preparation of the compositions of this invention in order to get adequate "wetting" or flow of the polymer into the reinforcing material.
In J. Am. Chem. Soc., 112, pages 2399-2402, entitled "Remarkably Selective Formation of Macrocyclic Aromatic Carbonates: Versatile New Intermediates for the Synthesis of Aromatic Polycarbonates" (1990), the role of cyclic oligomers as intermediates in the preparation of aromatic polycarbonates is described. The journal article also describes the commercial importance of the use of ring-opening polymerization in the preparation of polyamides, aliphatic polyesters, silicones, and epoxide thermosets in the absence of reinforcing material.
Also, it is known in the art to polymerize cyclic oligomers of ethylene terephthalate in the absence of reinforcing material. Japanese Patent Publication 48-33277 (Publication Date: Oct. 12, 1973) to Nihon Ester KK, and Polymer, Volume 1, pages 384-396 (1960). However, the prior art does not show the combination of oligomers with reinforcing materials to attain good wetting prior to polymerization.